During certain well drilling processes, it may be desirable to communicate information from the bottom of the wellbore to the surface. For instance, logging-while-drilling (LWD) and measurement-while-drilling (MWD) techniques may generally include the collection of a number of various measurements via one or more sensors within the wellbore. Data collected through such techniques may include measurements related to characteristics of the wellbore (e.g., azimuth and inclination) or drilling components (e.g., rotational speed) themselves, or measurements pertaining to the properties of geologic formations (e.g., density, pressure, or resistivity) proximate the wellbore, for example.
The measured data may be communicated to the surface through mud pulse telemetry techniques, in which drilling fluid or “mud” is used as a propagation medium for a signal wave, such as a pressure wave. More specifically, data may be communicated by modulating one or more features of the wave to represent the data. For instance, the amplitude, the frequency, and/or the phase of the wave may be varied such that each variation represents either a single data bit (i.e., binary modulation) or multiple data bits (i.e., non-binary modulation) of digital data. As the wave propagates to the surface, these modulations may be detected and the data bits may be determined from the modulations.
It is noted, however, that the characteristics of the downhole modulator used and the mud pulse telemetry channel itself may impact communication rates, power, bandwidth, and accuracy of various modulation techniques. For instance, in a phase shift keying (PSK) modulation technique digital data is generally impressed onto the wave in the mud by modulating the phase of the wave from within the wellbore. A demodulator at the surface detects the phase and reconstructs the digital data.
While PSK modulation generally calls for abrupt (in fact, instantaneous in the ideal case) changes of phase, it will be appreciated by those skilled in the art that the above-described modulator cannot generate instantaneous phase changes. Instead, mud pulse telemetry systems employing PSK modulation typically approximate the abrupt phase changes by making phase changes to the wave as quickly as mechanically allowed by the downhole modulator. Although controlling the modulator to implement phase changes as quickly as physically possible does enable data to be communicated via certain lower-order PSK techniques (e.g., binary PSK), it is believed that such control does not effectively allow data to be communicated via other higher-order PSK techniques (e.g., 8-PSK, in which eight discrete phases are used to represent various data groups having three bits each).